1. Field of the Invention
The present invention relates generally to high-voltage direct current (HVDC) transmission systems, and more particularly to a gas circuit arrangement interrupting a DC current in a line of an HVDC transmission system. The invention also relates to a capacity setting method for determining the capacity of a parallel reactor and a parallel capacitor for use in the circuit of this arrangement.
2. Description of the Related Art
Recently, as power systems require higher voltages, the circuit breakers adapted therein become more critical in the achievement of further enhanced current interruption performance. At present, the arrangement for a gas circuit breaker with a reactor and capacitor connected in series are becoming more widely used in interrupting DC line power systems. One of the presently available circuit breakers has been described, for example, in "Journal of Power-Energy Division Conference 1994 of the Institute of Electrical Engineers", No. 621, pp. 824-825. The circuit configuration of such an arrangement for a DC circuit breaker device is illustrated in FIG. 21, wherein the device includes a DC circuit breaker 1, a parallel impedance means consisting of a parallel reactor 2 and a parallel capacitor 3, an energy-absorbing element 4 connected in parallel with the series circuit of parallel capacitor 3 and parallel reactor 2 for absorbing any excess voltage (overvoltage) at the parallel capacitor 3, and a DC current carrying line 5 in a power system. The energy-absorbing element may alternatively be connected to the parallel capacitor 3 only.
The DC circuit breaker 1 is constituted by a presently available puffer type gas circuit breaker, the cross-section of which is illustrated in FIG. 22. The gas circuit breaker has a pair of contacts: a fixed contact 11 to allow the flow of the DC current of the device, and a movable contact 14 in a puffer cylinder 12 with a dielectric nozzle 13 fixed thereto. In the open state, an arc 17 is generated between the contacts 11, 14 when a piston rod 16 integrated with the movable contact 14 is moved with respect to the puffer piston 15 secured to the fixed contact 11. At this time, as the piston rod 16 moves, an arc-extinguishing gas 18, here SF.sub.6, filled within the inner space defined by the movable contact 14, the puffer cylinder 12 and the puffer piston 15 is compressed to be sprayed onto the arc 17 through an opening 19.
The prior art device operates as follows. When the fixed contact 11, which carries the DC current of the puffer type gas circuit breaker, and the movable contact 14 are open-circuited, an arc 17 is generated between these contacts in substantially the same manner as in the alternate current (AC) interrupted state. In the case of DC current, however, simply spraying the DC arc with SF.sub.6 gas may not be sufficient to interrupt and extinguish it successfully due to the fact that, unlike AC current, DC current does not periodically cross the current zero point.
To extinguish the arc, the parallel reactor 2 and the parallel capacitor 3 are thus coupled in parallel to the DC circuit breaker 1 causing the current to be commutated and also causing the arc current to oscillate to come closer to the current zero point. This permits the SF.sub.6 gas 18 compressed by the puffer piston 15 to be blown out from the opening 19 and then sprayed against the arc 17 through the dielectric nozzle 13 thus forcing it to be extinguished.
A significant problem with the prior arrangement for a DC gas circuit breaker is that, while the parallel reactor and the parallel capacitor for commutation may play an important role in attaining amplification of the perturbation of the arc current, how to appropriately determine the exact values for these depending upon the actual DC interruption current value and the performance of DC circuit breaker employed still remains unknown.
Another problem of the prior art is that the method for setting the capacity is yet unknown in terms of determination of suitable reactance values of the capacitor and reactor used in the circuit breaker.